1. Field of the Invention
This invention relates broadly to production of oil from formations. More particularly, this invention relates to dual completion systems and related methods for segregating oil and water downhole so that oil is produced separately from water.
2. State of the Art
At some time in the life of most wells, water is co-produced with hydrocarbons (e.g., oil). Lifting this water to the formation surface, separating it from the hydrocarbons, cleaning, and disposing it contribute to the costs of production operations. Moreover, because of issues related to two-phase flow (e.g. water and oil) in the wellbore, lifting of water along with oil may substantially reduce overall oil production. In addition, in some cases, regulatory restrictions call for treatment of water once it is brought up to the surface. Because of these considerations, various downhole oil-water separation (DHOWS) schemes have been proposed. None has been universally successful since all have technical and/or economical limitations.
One DHOWS scheme utilizes hydrocyclones. Reservoir simulation and economics studies suggest that DHOWS systems should be installed soon after water breakthrough when oil flow rates are still high and water rates low. Hydrocyclones, however, are not well-suited to situations in which the produced water fraction is small. In addition, hydrocyclones are often unreliable, particularly downhole.
In-well gravity separation is another DHOWS scheme which has been proposed. However, in-well gravity separation will not perform well at low water cuts or high water cuts, and therefore, this technique has not gained traction in the art. They also do not perform well for high flow rates due to viscous drag, and high crude oil viscosity where separation may be hindered.
Dual-drain completions that attempt to separately produce fluids that are already largely segregated by gravity within the reservoir have been proposed as a DHOWS scheme. See, e.g., co-owned U.S. Pat. No. 6,415,864 to Ramakrishnan et al. These completion systems work in conjunction with reservoir monitoring and control of the oil-water transition zone. In particular, these systems are best deployed with monitoring via a sensing mechanism for the oil-water transition around the wellbore. Because this is technologically intensive, costs are large, if not prohibitive.
A fourth proposed DHOWS scheme is disclosed in U.S. Pat. No. 4,296,810 to Price where membranes are suggested for oil-water separation. While membranes can facilitate the delay of the onset of water production, this approach is limited since naturally, a near-wellbore water-block will form and reduce oil production or allow water breakthrough to occur. Furthermore deployment of relatively fragile membranes while maintaining integrity has not been overcome.